Combine the butter, onion, and chiles in a saute pan over medium heat and cook until the onion softens and becomes translucent, about 5 minutes.,Stir in the tomato and oregano and cook for 1 minute.,Stir in the cream, buttermilk, and flour and bring to a simmer to thicken slightly, about 3 minutes.,______,Season to taste with salt and pepper.,Serve immediately, topped with the queso fresco.